Python reverse / invert a mapping

Question

I have this dictionary:

map_old = { 'p': 8, 'q':9 }

How can I invert this map to get:

 

map_inv = { 8: 'p', 9: 'q' } 

Answer 1

There are various ways of doing this, some of them are a follows:

1. Inverting with map and reversed

map_inv = dict(map(reversed,map_old.items()))

2.Inverting with a comprehension

For Python 2.7.x use:

map_inv = {v: k for k, v in map_old.iteritems()} 

For Python 3 and higher version use:

map_inv = {v: k for k, v in map_old.items()}

3.Inverting with a Defaultdict

 from collections import defaultdict

map_inv = defaultdict(list)

{map_inv[v].append(k) for k, v in map_old.items()}

4.Inverting with a for loop:

map_inv = dict()

for key, value in map_old.items():

map_inv.setdefault(value, list()).append(key)

 

The preferable way is by using the dictionary comprehension but it has a constraint that it cannot work with non-unique values. So, if you are working with non-unique values then refer to the other methods.

Hope it helps!